1. Field of the Invention
This invention relates to power transmissions driven from an automotive engine which transmits power to the output shaft over a continuously variable stepless range of speed and torque multiplication. More particularly, the invention pertains to a continuously variable belt drive wherein the conventional start-up clutch is absent, but a torque converter or fluid coupling is driven from the continuously variable drive unit.
2. Description of the Prior Art
A continuously variable speed ratio transmission transmits power from an input shaft to an output shaft over a range of speed ratios that varies steplessly within a given range. A device for producing this result may transmit power between the shafts by an endless, flexible belt or chain driven by a variable diameter pulley, the belt driving a second variable diameter pulley mounted on the output shaft.
Stepless variable speed ratio transmissions generally include a torque converter arranged to be driven directly from the engine. The continously variable device is driven by the torque converter and is followed by conventional gearing for producing a further speed reduction and a directional change between the sense of direction of the engine crankshaft and the output shaft of the transmission. An example of a transmission organized in this way is shown in U.S. Pat. No. 3,526,305.
This arrangement has two disadvantages, the first is that the transmission is longer in a direction parallel to the pulley shafts than is necessary. This results because the torque converter, the change direction gearing and one pulley of the belt drive are arranged to rotate about the same axis. A second disadvantage is evident upon stopping the vehicle under an emergency condition when the transmission is operating at the high speed ratio end of its available range. Continuously variable speed drives usually change speed ratio only when the drive mechanism is rotating, therefore, if the transmission is stopped while operating in the high end of its speed ratio range, the vehicle must be started in the high speed ratio which is at a distinct disadvantage.
Other arrangements of the principal drive elements of a transmission of the variable speed ratio type include a hydrokinetic torque converter driven directly from the engine of the vehicle followed by a continuously variable drive that may be a belt-pulley combination or a traction drive, and a gear set for producing a reverse direction to the output shaft. U.S. Pat. No. 4,125,037 shows a drive system having these components arranged in this sequence. Other arrangements of this transmission have the continuously variable drive located directly behind the engine, a torque converter located on the driving pulley shaft of the continuously variable drive and a reverse drive gear set immediately behind the driven pulley of the continuously variable speed drive. A third arrangement of the principal elements located for use with an engine transversely mounted in the vehicle has the torque converter driven from the engine, a traction drive located immediately behind and driven from the torque converter, and a reversing gear set whose output parallels the axis of the output shaft of the engine and transmits power directly to the drivewheels of the vehicle.
Frequently belt driven transmissions employ a centrifugal start-up clutch in the drive system, which acts to disengage the engine from the transmission until engine speed has increased sufficiently to engage the clutch to absorb the inertia of the transmission unit. This result, although an advantage in starting the engine from rest, raises the possibility that the transmission will disengage from the engine when the vehicle passes from cruise speed to a coast condition as frequently occurs in city driving. Furthermore, the inclusion of a start-up clutch of the centrifugal type would prevent fast but gentle rocking motions needed on ice and snow. When the engine is on fast idle as in cold starting conditions the centrifugal clutch tends to engage prematurely, but a hydrokinetic device would smooth the engagement avoiding abrupt torque application to the drive wheels.